Piston pump

ABSTRACT

A piston pump which is intended particularly for use in a slip-controlled hydraulic vehicle brake system. To increase a pumping volume of the piston pump at low pumping pressure, the piston pump includes a body embodied preferably as an annular body which is attached axially displaceably to the piston. The body increases a cross- sectional area of the piston and is axially acted upon by a spring element. At low pressure, the piston pump pumps with the entire cross-sectional area of the piston and the body while conversely at a higher pumping pressure, the body is held back counter to the force of the spring element and does not move together with the piston so that then the piston pumps only with its own cross-sectional area. The piston pump of the invention has the advantage of improved pressure buildup dynamics when the pumping pressure at the onset of pumping is low.

This application is a 371 of PCT/DE98/02173 filed Jul. 30, 1998.

PRIOR ART

The invention relates to a piston pump which is intended in particularfor use as a pump for a hydraulic vehicle brake system.

One such piston pump is known for instance from German Patent DisclosureDE 41 07979 A1. The known piston pump has a piston which is receivedaxially displaceably in a pump housing and can be driven to execute areciprocating stroke motion in the axial direction by means of aneccentric element that can be driven to rotate by an electric motor.

ADVANTAGES OF THE INVENTION

The piston pump of the invention has, in addition to the piston, a bodythat just like the piston is received axially displaceably in the pumphousing. The body is furthermore axially displaceable relative to thepiston counter to the force of a spring element which is supported onthe piston. The piston can not only move together with the body axiallyin the pump housing, but can also move axially in the pump housing andrelative to the body when the body is in repose in the pump housing. Itis also possible for the piston and the body to move at different speedsand for different distances in the pump housing. The body increases across-sectional area of the piston. If the body in a stroke motion moveswith the piston, the pumping volume per piston stroke of the piston pumpof the invention increases in accordance with the cross-sectional areaof the body. When the body is in repose in the pump housing, the pumpingvolume per piston stroke is correspondingly less. The body movestogether with the piston whenever a force which a fluid pumped by thepiston pump exerts on the body by reason of its pressure is no greaterthan the force of the spring element which is braced on the piston andpresses axially against the body. Conversely, if the pressure of thepumped fluid is so high that the force exerted on the body by the fluidexceeds the force of the spring element, braced on the piston, atmaximum force of the spring element, then the body does not move in thepump housing. If the pressure of the pumped fluid is between these twopressures, then the body moves by a shorter axial distance than thepiston, and a fluid quantity is pumped that is greater than the pumpedfluid quantity when only the piston moves and is less than the fluidquantity when the body moves together with the piston.

The piston pump of the invention has the advantage that at low pumpingpressure, it pumps an increased fluid volume per piston stroke. Since adrive motor of the piston pump is designed for high pressure, the drivemotor is not overloaded by the increased pumping quantity at lowpressure. If the piston pump is pumping at high pressure, its pumpingvolume per piston stroke is reduced. As a result, the force required todrive the piston is reduced at the same time. The piston pump of theinvention thus has the advantage of a pumping volume that is dependenton the pumping pressure. When the piston pump of the invention is usedin a hydraulic vehicle brake system, this has the advantage of improvedpressure buildup dynamics: For instance, if in order to initiatebraking, pressure first has to be built up in wheel brake cylinders,then the piston pump of the invention, without changing its drive, pumpsan increased volumetric flow and thus assures a rapid pressure buildup.When a high final pressure is reached, or for instance in slip controlif after a limited pressure reduction in a wheel brake cylinder thepressure has to be increased again, then the pumping volume of thepiston pump of the invention is reduced, so that with a predeterminedaxial force on the piston, the pump pressure is increased.

In a preferred feature of the invention, the body is embodied as anannular body which is axially displaceable in the manner of a slidingsleeve on the piston and is also axially displaceable in the pumphousing. Because the body is embodied as an annular body, the inventioncan be realized in a simple way.

In particular, in one feature of the invention, a sealing and/or guidering that is necessary anyway is employed as the annular body that isjointly moveable with the piston.

This feature of the invention has the advantage of not requiring anyadditional body.

The piston pump of the invention is intended in particular as a pump ina brake system of a vehicle and is used to control the pressure in wheelbrake cylinders. Depending on the type of brake system, theabbreviations ABS, ASR, FDR and EHB are used for such brake systems. Inthe brake system, the pump serves for instance to return brake fluidfrom a wheel brake cylinder or a plurality of wheel brake cylinders to amaster cylinder (ABS) and/or to pump brake fluid out of a supplycontainer into a wheel brake cylinder or a plurality of wheel brakecylinders (ASR or FDR or EHB). In a brake system with wheel slip control(ABS or ASR) and/or a brake system serving as a steering aid (FDR)and/or an electrohydraulic brake system (EHB), the pump is needed. Withthe wheel slip control (ABS or ASR), locking of the wheels of thevehicle during a braking event involving strong pressure on the brakepedal (ABS) and/or spinning of the driven wheels of the vehicle in theevent of strong pressure on the gas pedal (ASR) can for instance beprevented. In a brake system serving as a steering aid (FDR), a brakepressure is built up in one or more wheel brake cylinders independentlyof an actuation of the brake pedal or gas pedal, for instance to preventthe vehicle from breaking out of the track desired by the driver. Thepump can also be used in an electrohydraulic brake system (EHB), inwhich the pump pumps the brake fluid into the wheel brake cylinder orwheel brake cylinders if an electric brake pedal sensor detects anactuation of the brake pedal, or in which the pump is used to fill areservoir of the brake system.

DRAWING

The invention will be described in further detail below in terms of anexemplary embodiment shown in the drawing.

FIG. 1 shows an axial section through a piston pump of the invention;

FIG. 2 shows a schematic pumping characteristic curve graph;

FIG. 3 shows a detail of a modified embodiment of the piston pump of theinvention indicated by the arrow III in FIG. 1.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The piston pump 10 of the invention, shown in FIG. 1, is inserted into ahydraulic block 12, of which the drawing shows only a fragmentsurrounding the piston pump 10. Other hydraulic components, not shown,such as magnet valves, hydraulic reservoirs and dampers of aslip-controlled vehicle brake system, are inserted into the hydraulicblock 12 and are hydraulically connected to one another and to thepiston pump 10 by the hydraulic block 12. The hydraulic block 12 forms apump housing of the piston pump 10 of the invention and will hereinafterbe called the pump housing 12.

A tubular bush 16 is press-fitted into a stepped continuous bore 14 inthe pump housing 12. A pin-like piston 18 is received axiallydisplaceably in the bush 16. The piston 18 protrudes for a portion ofits length from the bush 16. On its end protruding from the bush 16, thepiston 18 is guided axially displaceably in the pump housing 12 by aguide ring 20 and is sealed off in the pump housing 12 by a sealing ring22. The guide ring 20 and the sealing ring 22 are inserted jointly intoa groove that is made in the bore 14 in the pump housing 12.

One end of the piston 18, located in the bush 16, is axiallydisplaceably guided in the bush 16 by means of a guide ring 24 and issealed off in the bush 16 by means of a sealing ring 26; both rings areslipped onto the piston 18. The guide ring 24 and the sealing ring 26are axially displaceable both in the bush 16 and on the piston 18. Thedisplacement travel in one direction is limited by a radial flange 28 ofa valve cage 30, which is mounted on the face end of the piston 18 thatis located in the bush 16, and whose radial flange 28 protrudes radiallyoutward from a circumference of the piston 18. The radial flange 28forms an axial stop of the piston 18 for the guide ring 24 and thesealing ring 26.

In the opposite direction, the displacement travel of the guide ring 24and sealing ring 26 in the bush 16 is limited by an annular shoulder 32,which reduces a diameter of the bush 16 on the face end from which thepiston 18 protrudes for part of its length. The annular shoulder 32forms an axial stop of the bush 16 for the guide ring 24 and the sealingring 26 which are axially displaceable in the bush 16 and on the piston18.

The guide ring 24 and the sealing ring 26 are pressed by a helicalcompression spring 34, which is slipped onto the piston 18, against theradial flange 28 of the valve cage 30 forming the axial stop of thepiston 18. The helical compression spring 34 is supported on the piston18 via a shim ring 36 and a snap ring 38, which is inserted into apiston groove. A support ring 40 is placed between the helicalcompression spring 34 and the guide ring 24. The helical compressionspring 34 presses the guide ring 24 and the sealing ring 26 with initialtension against the radial flange 28, on the piston 18.

The guide ring 24 and sealing ring 26 form a body which is axiallydisplaceable both in the bush 16 and on the piston 18; the guide ring 24and sealing ring 26 will therefore be referred to hereinafter as thebody 24, 26. The body 24, 26 increases a cross-sectional area of thepiston 18. The support ring 40 can also be considered as a component ofthis body.

The piston pump 10 of the invention has a positive displacement chamber42 inside the bush 16, which is defined by the piston 18, with the body24, 26 axially displaceable on the piston, and by a closure plug 44,which is inserted into a face end of the bush remote from the annularshoulder 32.

The body 24, 26 that is axially displaceable on the piston 18 has thefollowing function: If a pressure that exerts a force on the body 24, 26and that is less than the prestressing force of the helical compressionspring 34 prevails in the positive displacement chamber 42, then thehelical compression spring 34 keeps the body 24, 26 in contact with theradial flange 28; that is, the piston 24, 26 moves together with thepiston 18. This means that upon a pumping stroke of its piston 18, inwhich the piston 18 moves into the bush 16, the piston pump 10 of theinvention positively displaces a fluid volume (pumping volume) out ofthe positive displacement chamber 42 that is equivalent to the productof the travel of the piston 18 in the pumping stroke (piston stroke) andan internal cross-sectional area of the bush 16. That is, a largevolumetric flow is pumped.

Conversely, if a pressure that keeps the body 24, 26 on the annularshoulder 32 of the bush 16 counter to the force of the helicalcompression spring 34 prevails in the positive displacement chamber,then the piston 18 in its pumping stroke positively displaces a volumefrom the positive displacement chamber 42 that is equivalent only to theproduct of the piston stroke and the cross-sectional area of the piston18, which is less than the internal cross- sectional area of the bush 16by the area of, an annular end face of the body 24, 26 located betweenthe piston 18 and the bush 16. If the pressure in the positivedisplacement chamber 42 is between the two pressures described, then thebody 24, 26 is displaced in the bush 16 by a shorter distance than thepiston stroke of the piston 18, and correspondingly the pumping volumeis between the two cases described. Thus, at a low pumping pressure, thepiston pump of the invention has an increased pumping volume.

A pumping characteristic curve of the piston pump 10 of the invention isshown schematically, compared with a pumping characteristic curve of aconventional piston pump, in the graph in FIG. 2. A mean pumping volumeflow is plotted on the ordinate, and the pumping pressure, that is, thepressure difference between the pump inlet and the pump outlet, isplotted on the abscissa. The upper pumping characteristic curve 44pertains to the piston pump 10 of the invention, while the lower pumpingcharacteristic curve 46 pertains to a conventional piston pump. Theincreased pumping volume flow of the piston pump 10 of the invention atlow pumping pressure is clearly shown.

The prestressing force of the helical compression spring 34 isequivalent to a pressure of approximately 20 to 30 bar in the positivedisplacement chamber 42 of the piston pump 10. In FIG. 1, the terminalposition of the piston at the end of the pumping stroke is shown, thatis, when the piston 18 has been thrust the farthest inward into the bush16.

For fluid admission into the piston pump 10 of the invention, the piston18 has an axial blind bore 48, which is intersected by transverse bores50. The fluid admission is effected through an inlet bore 52 in the pumphousing 12, which discharges radially into the stepped bore 14, and fromthere on into the end, remote from the positive displacement chamber 42,of the bush 15, and into the transverse bores 50 and the blind bore 48of the piston 18. The blind bore 48 discharges with a conical valve seat54 into the positive displacement chamber 42. The valve seat 54, likethe valve cage 30, is part of an inlet valve 56 of the piston pump 10embodied as a spring-loaded check valve 56. This inlet valve 56 has avalve ball 58 as the valve closing body, which is pressed against thevalve seat 54 by a helical compression spring 60 acting as a valveclosing spring. The valve closing spring 60 and the valve ball 58 arereceived in the valve cage 30. The valve cage 30 is a cup-shapeddeep-drawn sheet-metal part with flow openings 62 on its circumferenceand its bottom. The valve cage 30 has an annular shoulder 64 on its openside, with which it rests on the face end, located in the bush 16, ofthe piston 18. It is held in contact with the piston 18 by a pistonrestoring spring 66 embodied as a helical compression spring, which isinserted into the positive displacement chamber 42, is braced againstthe closure plug 44, and presses against the radial flange 28 of thevalve cage 30. The piston restoring spring 66 presses the piston 18against a circumference of an eccentric element 67, which can be drivenby an electric motor and serves in a manner known per se to drive thepiston 18 to execute its reciprocating motion.

As its outlet valve 68, the piston pump 10 also has a spring-loadedcheck valve, which is accommodated in the closure plug 44. The closureplug 44 has an axial blind bore 70, which is opened toward the positivedisplacement chamber 42 and into which a helical compression springacting as a valve closing spring 72 is inserted; this spring presses avalve ball 74, received in the blind bore 70 and acting as a valveclosing body against a conical valve seat 76 of the outlet valve 68. Thevalve seat 76 is mounted on a circular-annular valve seat body 78, whichis inserted from a face end toward the positive displacement chamber 42into the closure plug 44 and is held by a caulk 80.

Fluid emission is effected through a radial bore 82 in the closure plug44, via an annular conduit 84 surrounding the closure plug 44, into aradial outlet bore 86 in the pump housing 12.

The closure plug 44 is press-fitted in fluid-type fashion into anannular shoulder-shaped enlargement 88 of the bush 16. The closure plug44 is held in the pump housing 12 by a calk 90 of the pump housing 12and closes the stepped bore 14 in the pump housing 12 in pressure-typefashion.

The piston pump 10 shown can also be modified as needed in such a waythat the helical compression spring 34 is braced not on the piston 18but rather on the pump housing 12, for instance in the region of theguide and sealing ring 20, 22. In this modification, not shown, the shimring 36 and snap ring 48 can be dispensed with. However, it is a certaindisadvantage of this modification that the helical compression spring 34must be designed for a longer stroke, in comparison with the versionshown in the drawing.

The modification, shown only as a detail in FIG. 3, of the piston pumpshown in FIG. 1 has an annular body 92 of S-shaped annular crosssection, in the form of a body that is axially displaceable in the pumphousing 12 and relative to the piston 18. The annular body 92 comprisesa dimensionally stable plastic that slides easily and guides well, suchas the same material as the guide ring 24 of the piston pump 10 shown inFIG. 1. The annular body 92 is slipped axially displaceably onto thepiston 18; it rests in an annular interstice between the piston 18 andthe bush 16. The annular body 92, viewed in the axial direction, islocated between the helical compression spring 34 and the radial flangeof the valve cage 30; the annular body 92--like the guide ring 24 andsealing ring 26 in the piston pump shown in FIG. 1--is pressed againstthe radial flange 28 by the helical compression spring 34 which is underinitial tension and if there is a sufficient pressure difference betweenthe positive displacement chamber and an inlet side of the piston pump,it is lifted from the radial flange 28 counter to the force of thehelical compression spring 34.

Two sealing rings 94, 96, are located in two encompassing grooves of theannular body 92 that are formed by the S-shaped annular cross section ofthe annular body 92, one of the grooves opening outward and the otheropening inward; of these sealing rings, the sealing ring 94 providessealing between the annular body 92 and the bush 16, and the othersealing ring 96 provides sealing between the annular body 92 and thepiston 18. Using the dimensionally stable annular body 92 has theadvantage that the annular body 92 absorbs the axial force exerted bythe helical compression spring 34; the sealing rings 94, 96 resting inthe annular body 92 are not acted upon by the force of the helicalcompression spring 34. This improves the durability of the sealing rings94, 96, reduces their friction, and improves their sealing action. Thedimensionally stable annular body 92 brings about optimal installationconditions for the sealing rings 94, 96. The annular body 92 guides thepiston 18 axially in the bush 16, so that no separate guide ring isnecessary. Together with the sealing rings 94, 96, the annular body 92provides sealing between the piston and the bush 16, so that the sealingring 26 of the piston pump 10 shown in FIG. 1 is omitted. The supportring 40 of the piston pump shown in FIG. 1 is equally unnecessary.

Otherwise, the detail in FIG. 3 showing a modification according to theinvention of the piston pump 10 shown in FIG. 1 agrees with that pistonpump 10 and functions in the same way. The same reference numerals areused for components that match. To avoid repetition, in this respect seethe above descriptions of FIGS. 1 and 2.

The foregoing relates to the preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed is:
 1. A piston pump for a vehicle brake system,comprising:a pump housing (12) and a piston (18) that is driven toexecute a reciprocating stroke motion and is received axiallydisplaceably in a bore (14) in the pump housing (12); a fluid inlet (52)and a positive displacement chamber (42) formed in the pump housing; abody (24, 26, 40; 92 94, 96), having at least a first portion and asecond portion, axially displaceably received in the pump housing (12),the first portion being positioned closer to the positive displacementchamber (42) than the second portion, wherein the body (24, 26, 40, 92,94 96) is axially displaceable relative to the piston (18) and reacts toa force of a spring element (34), the body increasing a cross sectionalarea of the piston (18), and wherein the fluid inlet (52) is positionedcloser to the second portion of the body than to the first portion ofthe body (24, 26, 40; 92, 94, 96), and discharges into the bore (14);and a fluid opening (48, 50) is provided in the piston and connects thefluid inlet (52) to the positive displacement chamber (42).
 2. Thepiston pump according to claim 1, in which a check valve (56) isprovided in the fluid opening (48, 50) to open and close access to thedisplacement chamber.
 3. The piston pump according to claim 2, in whichthe check valve (56) in the fluid opening (48, 50) allows a fluid flowonly from the fluid inlet (52) to the positive displacement chamber(42).
 4. The piston pump according to claim 1, in which the springelement (34) is supported on the piston (18).
 5. The piston pumpaccording to claim 2, in which the spring element (34) is supported onthe piston (18).
 6. The piston pump according to claim 3, in which thespring element (34) is supported on the piston (18).
 7. The piston pumpaccording to claim 1, in which the body is an annular body (24, 26, 40;92, 94, 96) slipped onto the piston
 18. 8. The piston pump according toclaim 2, in which the body is an annular body (24, 26, 40; 92, 94, 96)slipped onto the piston
 18. 9. The piston pump according to claim 3, inwhich the body is an annular body (24, 26, 40; 92, 94, 96) slipped ontothe piston
 18. 10. The piston pump according to claim 4, in which thebody is an annular body (24, 26, 40; 92, 94, 96) slipped onto the piston18.
 11. The piston pump according to claim 7, in which the annular bodyhas a sealing ring (26, 94, 96).
 12. The piston pump according to claim8, in which the annular body has a sealing ring (26, 94, 96).
 13. Thepiston pump according to claim 9, in which the annular body has asealing ring (26, 94, 96).
 14. The piston pump according to claim 10, inwhich the annular body has a sealing ring (26, 94, 96).
 15. The pistonpump according to claim 7, in which the annular body has a guide ring(24, 92).
 16. The piston pump according to claim 11, in which theannular body has a guide ring (24, 92).
 17. The piston pump according toclaim 1, in which the spring element (34), against whose force the body(24, 26, 40; 92, 94, 96) is axially displaceable relative to the piston(18), has an initial tension.
 18. The piston pump according to claim 2,in which the spring element (34), against whose force the body (24, 26,40; 92, 94, 96) is axially displaceable relative to the piston (18), hasan initial tension.
 19. The piston pump according to claim 3, in whichthe spring element (34), against whose force the body (24, 26, 40; 92,94, 96) is axially displaceable relative to the piston (18), has aninitial tension.
 20. The piston pump according to claim 4, in which thespring element (34), against whose force the body (24, 26, 40; 92, 94,96) is axially displaceable relative to the piston (18), has an initialtension.
 21. The piston pump according to one claim 1, in which thepiston pump (10) has an axial stop (32), which is stationary with thepump housing (12), for the body (24, 26, 40; 92, 94, 96), and the axialstop (32) limits an axial motion of the body (24, 26, 40; 92, 94, 96)relative to the pump housing 12 in a direction that tenses the springelement (34) that acts on the body (24, 26, 40; 92, 94, 96).
 22. Thepiston pump according to one claim 2, in which the piston pump (10) hasan axial stop (32), which is stationary with the pump housing (12), forthe body (24, 26, 40; 92, 94, 96), and the axial stop (32) limits anaxial motion of the body (24, 26, 40; 92, 94, 96) relative to the pumphousing 12 in a direction that tenses the spring element (34) that actson the body (24, 26, 40; 92, 94, 96).
 23. The piston pump according toone claim 3, in which the piston pump (10) has an axial stop (32), whichis stationary with the pump housing (12), for the body (24, 26, 40; 92,94, 96), and the axial stop (32) limits an axial motion of the body (24,26, 40; 92, 94, 96) relative to the pump housing 12 in a direction thattenses the spring element (34) that acts on the body (24, 26, 40; 92,94, 96).
 24. The piston pump according to one claim 4, in which thepiston pump (10) has an axial stop (32), which is stationary with thepump housing (12), for the body (24, 26, 40; 92, 94, 96), and the axialstop (32) limits an axial motion of the body (24, 26, 40; 92, 94, 96)relative to the pump housing 12 in a direction that tenses the springelement (34) that acts on the body (24, 26, 40; 92, 94, 96).
 25. Thepiston pump according to claim 1, in which the piston (18) has an axialstop (28), which limits an axial displacement travel of the body (24,26, 40; 92, 94, 96) relative to the piston (18) in a direction in whichthe spring element (34) that acts on the body (24, 26, 40; 92, 94, 96)is relieved.
 26. The piston pump according to claim 2, in which thepiston (18) has an axial stop (28), which limits an axial displacementtravel of the body (24, 26, 40; 92, 94, 96) relative to the piston (18)in a direction in which the spring element (34) that acts on the body(24, 26, 40; 92, 94, 96) is relieved.
 27. The piston pump according toclaim 3, in which the piston (18) has an axial stop (28), which limitsan axial displacement travel of the body (24, 26, 40; 92, 94, 96)relative to the piston (18) in a direction in which the spring element(34) that acts on the body (24, 26, 40; 92, 94, 96) is relieved.
 28. Thepiston pump according to claim 4, in which the piston (18) has an axialstop (28), which limits an axial displacement travel of the body (24,26, 40; 92, 94, 96) relative to the piston (18) in a direction in whichthe spring element (34) that acts on the body (24, 26, 40; 92, 94, 96)is relieved.